1. First female musical artist to have a simultaneous #1 film and #1 album, familiarly → JLO
4. Rebound, as a pool shot → BANK
8. Requirement for domestic air travel starting in 2025 → REALID
14. Body part that might "pop" → EAR
15. Words of understanding → ISEE
16. ___ State, nickname for New York → EMPIRE
17. Soften, as a message → TONEDOWN
19. Kitchen strainers → SIEVES
20. "Don't go!" → STAY
21. Appear gradually from the black, as a film image → FADEIN
23. Industrialist Lilly → ELI
24. Native led by Montezuma → AZTEC
26. A very high price, so to speak → TOPDOLLAR
28. Cereal variety from Switzerland → MUESLI
30. Lead female character → HEROINE
31. Summer temp, say → INTERN
34. River's bottom → BED
35. Dentist, colloquially → TOOTHDOCTOR
38. Actress Long of "Boyz N the Hood" → NIA
39. Gets back together → REUNES
40. Shipped without prepaid postage, for short → SENTCOD
42. Much of Mauritania and Mali → SAHARA
47. Japanese stadium that has hosted M.L.B. and N.F.L. games → TOKYODOME
50. Martian explorer → ROVER
51. Last number in a countdown → ONE
52. Surgical stitch → SUTURE
54. Used a loom → WOVE
55. Related to spiritual cause and effect → KARMIC
57. Collection of tasks … or what 17-, 26-, 35- and 47-Across comprise? → TODOLIST
59. Timeless, to a poet → ETERNE
60. Small vortex → EDDY
61. "Thanks, Captain Obvious!" → DUH
62. Best-of-seven games, say → SERIES
63. Make airtight → SEAL
64. Fancy hotel amenity → SPA